Improper ventilator settings during long-term ventilation may cause lung damage, known as ventilator induced lung injury (VILI). Limiting or eliminating VILI has been a topic of scientific discussion for years. As a result of this, two main damaging mechanisms have been identified: (1) shear stress caused by lung collapse at end-expiration and (2) over-distension causing tissue breakage allowing fluidic communication between alveolar space and lung capillaries.
To reduce VILI, lung-protective low tidal volume-high positive end-expiratory pressure (PEEP) ventilation strategies have been recommended. The high PEEP is used to prevent lung collapse. However, high PEEP distends the lung tissue, reducing the tidal volume range that can be used without overstretching the lungs. Therefore the strategy is accompanied with reduced tidal volume. Such strategies are also called permissive hypercapnia because low tidal volume may not maintain sufficient CO2 clearance from the circulation, causing blood CO2 content increase, i.e. hypercapnia. Hypercapnia is a non-normal physiological state in which negative side effects are possible. Therefore, optimal ventilation settings incorporate the lowest PEEP that prevents end-expiration collapse of the lungs to maximize the elastic lung volume range for the tidal volume.
Adjusting the ventilator for optimal PEEP for a particular patient is problematic since diagnostic real-time bedside measurements for lung collapse are missing. In state of the art therapy, common values based on statistical studies are used for patients classified to represent particular forms of lung sicknesses. From an individual patient point of view, such statistical values may be too low or too high compared to the optimal. Visual inspection for inflection point in the curvature of a graph where the breathing volume is presented as a function of breathing circuit pressure has been used. Locating this point is difficult and, when found, it may represent properties other than the end-expiratory lung collapse, such as properties of the chest wall, ventilator, breathing circuit, or endotracheal tube.
Although PEEP is very a common property of artificial ventilation and is effective in preventing lung collapse, it is not very effective in recruiting collapsed lung regions. For this purpose, recruitment manoeuvres are used wherein the lung is opened with one or more inspirations with increased tidal volume and duration. Also, patient positioning can be used to recruit the lungs. Prone or side positioning may open the lungs compared to more normal supine positions. Further forms of recruitment may be obtained by infusion of drugs, such as surfactants or bronchodilators. Clinical problems result however because direct measurements indicating the need for recruitment and its efficacy are missing at the bedside. Without such information, decisions have to be made based on indirect parameters such as oxygenation, which are not very sensitive for lung collapse.